Many additions were made to the original Norman structure over the years. The exterior was "Gothicized" in the 13th and 14th centuries, and the Spaniards made their mark in the 15th century.

But if anyone could be called the culprit for the cathedral's playground of styles, it is the Neapolitan architect Ferdinando Fuga, who went with the mood of his day and in 1771 and 1809 gave both the exterior and the interior of the Duomo a sweeping Neoclassical style. - in: http://www.sacred-destinations.com/italy/palermo-duomo